In order to improve safety when driving a vehicle, there has been proposed an AFS that controls the deflection of the irradiation direction of a headlamp in a cornering direction in response to the change in the steering angle of a steering wheel. According to this system, it is possible to make the irradiation direction of the headlamp face not only straight ahead but also in a direction where a driver steers the vehicle (i.e., a direction in which the vehicle is traveling). Therefore, this system is effective in improving safety in driving. Further, in recent years, as disclosed in JP-A-2005-29080, there has been proposed a device for controlling the deflection of a lamp. This device detects not only a steering angle but also detects the vehicle speed and controls the speed, responsiveness, and the like, at the time when the deflection of the irradiation direction of the headlamp is controlled, based on the vehicle speed and the steering angle. Accordingly, this device enables a headlamp to follow the change of conditions of a road on which the vehicle travels, and prevents a driver from feeling a sense of uneasiness.
In the device for controlling the deflection of a lamp using the AFS, an electric motor driven by an in-vehicle power source, such as an inexpensive stepping motor, has been used as a drive source. However, since the stepping motor easily controls a rotation angle but has a limitation on the maximum rotational speed, much time is required to change the irradiation direction of a lamp to a position that corresponds to a target deflection angle. For this reason, there is room for improvement in terms of the responsiveness of deflection control. Further, there are also problems in that rotation is not necessarily smooth, particularly the change of the irradiation range of a lamp is not smooth during the low-speed rotation. Accordingly, the employment of a DC brushless motor has been examined in recent years, instead of the stepping motor. The DC brushless motor has high maximum rotational speed and smooth rotation, and thus the DC brushless motor can quickly change the irradiation direction of the lamp to a position that corresponds to a target deflection angle. In addition, the DC brushless motor can smoothly change the irradiation range of the lamp as the deflection is controlled.
However, when the DC brushless motor is actually used for controlling the deflection of the irradiation direction of a lamp, the rotation characteristic of the DC brushless motor causes a problem in the deflection control. For example, in FIG. 9, the horizontal axis represents time, the vertical axis represents a steering angle and a deflection angle (referred to as a swivel deflection angle) of the irradiation direction of a headlamp, the dashed line shows a steering angle, and the thick solid line shows a swivel target angle that is a target value when a swivel deflection angle is controlled in response to the change in the steering angle. When the DC brushless motor is driven based on the swivel target angle in order to control the deflection of the headlamp, a difference angle between the swivel target angle and the actual swivel deflection angle at the time of control is calculated. The swivel deflection angle is set such that the difference angle is decreased, and the rotation of the DC brushless motor is controlled such that the irradiation direction of the lamp corresponds to the swivel target angle. Further, when the difference angle is decreased to a given angle and the swivel deflection angle thus is close to the swivel target angle, the rotation of the DC brushless motor is braked. Thus, control is performed such that the swivel deflection angle converges to the swivel target angle.
In the foregoing control, preferably the swivel deflection angle conforms to the swivel target angle. However, in fact, it is difficult to make the swivel deflection angle conform to the swivel target angle due to the rotation characteristic where the DC brushless motor can be driven at a high speed. Rotational speed is not constant and has an S-shaped characteristic as shown by a broken line in FIG. 9. That is, a slight delay occurs during the rising edge at the beginning of the rotation, and a suddenly accelerating operation is performed in order to compensate for the delay during the rising edge, so that the rotational speed becomes high. In addition, when the swivel deflection angle approaches the swivel target angle, sudden braking is performed so that the swivel deflection angle corresponds to the swivel target angle. For this reason, as for the actual swivel control, the deflection control where the irradiation direction is moved at a low speed is performed at the beginning of the deflection control, the deflection control where the irradiation direction is moved at a very high speed is performed thereafter, and the deflection control where the irradiation direction is rapidly decelerated is performed at the end of the deflection control. Therefore, the control where the moving speed of the irradiation direction is not stable is performed as a whole, so that a driver feels a sense of uneasiness. This is the same even in the case where the swivel deflection angle of the headlamp returns to the straight ahead direction as shown in FIG. 9. Further, this is not limited to the DC brushless motor, and is the same even in the case of using a motor capable of rotating at a high speed like the DC brushless motor.